If you’ve spent any time researching solar panel systems, you’ve likely come across both terms: irradiance and peak sun hours. They sound similar. They’re often used interchangeably. And that’s a problem, because they mean completely different things. Confusing them consistently leads to undersized or poorly performing solar systems.
This guide explains both terms clearly. It shows you how they relate to each other. It explains exactly how to use them when sizing a solar PV system. This is applicable whether you’re in Los Angeles, Johannesburg, Sydney, or anywhere else in the world.
The short answer: Irradiance is a measure of power (how intense sunlight is at a given moment, in W/m²). Peak sun hours is a measure of energy (the total daily solar energy available at a location, in kWh/m²/day). One is instantaneous. The other is cumulative.
What Is Solar Irradiance?
Irradiance is the measure of solar power hitting a surface at a specific instant in time. It is expressed in watts per square metre (W/m²).
Think of it this way: if you hold a solar panel up to bright midday sun on a clear day in the Karoo, the irradiance hitting that panel might be close to 1,000 W/m². The sun is at its peak intensity. That is roughly the amount of solar power available per square metre at that exact moment.
The internationally recognised standard for testing solar panels is the Standard Test Condition (STC). It defines an irradiance of exactly 1,000 W/m². This is why a 400W panel is rated at 400W: that’s its output under a precisely controlled 1,000 W/m² light source in a laboratory.
In reality, irradiance fluctuates constantly. It changes with:
- Time of day (low at sunrise and sunset, peaks at solar noon)
- Season (lower sun angle in winter reduces irradiance)
- Weather (cloud cover can drop irradiance to below 200 W/m²)
- Dust and soiling on the panel surface
- Altitude and atmospheric clarity
Irradiance is the real time intensity of sunlight. It tells you how hard the sun is shining right now, not how much solar energy is available over a full day.
What Are Peak Sun Hours?
Peak sun hours (PSH) is a different measurement altogether. It represents the total amount of solar energy received at a location during a day, expressed as the equivalent number of hours at full standard irradiance (1,000 W/m²).
If a location receives a total of 5,000 Wh of solar energy per square metre in a day, it accounts for morning ramp-up, midday peak, and afternoon decline. This equals 5 peak sun hours because 5 hours × 1,000 W/m² = 5,000 Wh/m².
Peak sun hours is an energy metric, not a power metric. It compresses an entire day’s worth of varying irradiance into a single, usable number for system sizing purposes.
Common mistake: Peak sun hours is not the number of hours of daylight. A location might have 12 hours of daylight but only 5.5 peak sun hours, because the sun isn’t at full intensity for all 12 of those hours.
How Irradiance and Peak Sun Hours Relate to Each Other
The relationship is straightforward once you understand the units:
| Metric | Unit | What it measures | Example |
|---|---|---|---|
| Irradiance | W/m² | Power: instantaneous sunlight intensity | 950 W/m² at solar noon |
| Solar energy (daily) | Wh/m²/day or kWh/m²/day | Energy: total daily solar resource | 6,200 Wh/m²/day |
| Peak sun hours | Hours/day | Energy expressed as equivalent hours at 1,000 W/m² | 6.2 PSH |
Peak sun hours and daily solar energy in kWh/m²/day are numerically identical. A location with 5.5 kWh/m²/day of daily solar radiation has 5.5 peak sun hours. The two are the same number expressed differently, one as energy, one as equivalent hours.
Below is a more graphical representation of these metrics.
Why This Distinction Matters for Solar System Sizing
This is where the practical importance becomes clear. When sizing a solar PV system, the calculation that drives everything is:
Required panel capacity (kWp) = Daily energy demand (kWh) ÷ Peak sun hours × System efficiency losses
If you confuse irradiance with peak sun hours, you’ll feed the wrong number into this equation and the result will either be a drastically oversized system (wasting money) or a critically undersized one (failing to meet energy demand).
Here’s a worked example:
A commercial building in Cape Town consumes 120 kWh per day. Cape Town receives approximately 5.5 peak sun hours on an annual average (accounting for seasonal variation). The system has efficiency losses of around 20% (inverter, wiring, soiling, temperature).
Required capacity = 120 ÷ 5.5 ÷ 0.80 = 27.3 kWp
If someone incorrectly used an irradiance figure of 950 W/m² as though it were peak sun hours, the calculation would produce a wildly incorrect result. The system would be sized completely wrong.
Peak Sun Hours Across South Africa and the World
South Africa is exceptionally well positioned for solar energy. The country receives some of the highest solar irradiance levels in the world, a significant competitive advantage for both residential and commercial solar adoption.
| Location | Average Peak Sun Hours (annual) | Notes |
|---|---|---|
| Northern Cape (Upington) | 5.5 – 6.5 PSH | Among the highest in the world |
| Johannesburg | 5.0 – 6.0 PSH | High plateau, excellent resource |
| Cape Town | 4.5 – 5.8 PSH | Seasonal variation — strong summers |
| Durban | 3.5 – 5.5 PSH | Humid subtropical — some cloud cover |
| Germany (Munich) | 3.0 – 3.5 PSH | Low but still viable with high feed-in tariffs |
| United Kingdom (London) | 2.5 – 3.0 PSH | Significant seasonal variation |
| Australia (Sydney) | 4.5 – 5.2 PSH | Strong solar resource |
| United States (Phoenix, AZ) | 6.0 – 6.5 PSH | Desert Southwest — world-class resource |
| India (Rajasthan) | 5.5 – 6.5 PSH | Rapidly expanding solar market |
These figures are annual averages. In practice, peak sun hours vary significantly by season. A Cape Town system will see 6.5+ PSH in December but as low as 3.5 PSH in June. This seasonal variation is one reason battery storage has become essential for year round energy security. Below is an image from Global Solar Atlas illustrating each location’s peak sun hours.
Where to Find Accurate Peak Sun Hours Data
For professional solar system design, you should always use verified irradiance data rather than estimates. The three most reliable sources are:
- NASA POWER — free global irradiance data at 0.5° resolution, updated regularly. Accessible at power.larc.nasa.gov
- Global Solar Atlas — World Bank tool providing GHI, DNI and PSH data globally. Excellent for quick reference at globalsolaratlas.info
- SolarGIS — professional grade irradiance data used in bankable yield assessments and project financing. Higher resolution than NASA POWER
- South African Weather Service (SAWS) — SA specific measurement station data for the most accurate local figures
When using solar design software like HelioScope or PVsyst, irradiance data from these sources is typically built in but always verify which data source the software is using before finalising a system design.
The Related Metrics You Need to Know: GHI, DNI and DHI
Peak sun hours figures are typically derived from Global Horizontal Irradiance (GHI). The total solar radiation received on a horizontal surface. But there are three distinct irradiance components used in solar design:
- GHI (Global Horizontal Irradiance) — total solar radiation on a horizontal surface. The most commonly referenced metric for comparing solar resources globally.
- DNI (Direct Normal Irradiance) — the component of solar radiation arriving directly from the sun, measured perpendicular to the sun’s rays. Critical for concentrated solar power (CSP) systems and for understanding how tracking systems perform.
- DHI (Diffuse Horizontal Irradiance) — solar radiation scattered by the atmosphere and clouds, arriving from all directions except directly from the sun. More important in cloudy climates where diffuse radiation makes up a larger share of total resource.
For standard flat panel rooftop and ground mounted PV systems, GHI is the primary metric used for sizing. For tilted systems, the relevant figure becomes GTI (Global Tilted Irradiance), what actually hits your panel surface at its specific tilt and azimuth angle.
Design tip: In South Africa, the optimal tilt angle for a fixed north facing system is approximately equal to your site’s latitude. A Johannesburg installation at 26°S would target around 25–28° tilt for maximum annual yield.
Practical Takeaways for Buyers, Installers and System Designers
Whether you’re specifying a system, reviewing a solar proposal, or checking an installer’s calculations, here’s what to take from this:
- Always check which metric your installer or designer is using. The sizing calculation must use peak sun hours (kWh/m²/day), not instantaneous irradiance (W/m²).
- Use annual average PSH for overall sizing, but model seasonal variation if battery storage or energy independence is a goal. A system sized for summer averages will underperform significantly in winter months.
- South Africa’s solar resource is world class. With 5.0–6.5 PSH across most of the country, the fundamental energy case for solar is stronger here than in Germany, the UK or most of Europe at a fraction of the grid electricity cost of those markets.
- Don’t rely on general figures for bankable projects. Any system above 50 kWp should use site specific irradiance data from SolarGIS or a verified measurement station, not regional averages.
Conclusion
Irradiance and peak sun hours measure different things. Irradiance is the power of sunlight at a given instant (W/m²). Peak sun hours is the total daily solar energy at a location, expressed as equivalent hours at full irradiance (kWh/m²/day). The two are related but they are not interchangeable, and using one where the other is needed produces meaningless calculations.
Getting this distinction right is the foundation of accurate solar system sizing. Whether you’re designing a 5 kW rooftop system for a home in Pretoria or a 5 MW ground mounted project in the the Middle East, the peak sun hours figure for your specific location is the single most important input in your energy yield model.
In the next article in this series, we break down GHI, DNI and DHI in detail explaining which radiation metric to use for different system types and how to read irradiance data from the Global Solar Atlas and NASA POWER tools.
Have a question about solar system sizing or irradiance data for your specific location? Leave a comment below or get in touch with the Green Future team.
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